1. Field of Endeavor
The invention relates to the design of an annular combustor of a gas turbine engine. More specifically the invention relates to a combustor design with reduced carbon monoxide emissions.
2. Brief Description of the Related Art
A gas turbine engine to which this invention can be applied is shown in FIG. 1 and has the following elements. A compressor 21 compressing air, for use in a high-pressure combustion chamber 22 is fitted with premix burners 20, as well as for cooling. Partially combusted air from the high-pressure combustor 22 passes through a high-pressure turbine 23 before flowing further into a low-pressure combustion 24 chamber where combustion occurs by self-ignition. In this chamber fuel is added to unburnt air from the high-pressure combustor 12 via a lance 37 that defines the burner of the low-pressure combustor 24. Both of the combustors include elongated toroidal shaped combustion chambers surrounding the shaft 30 of the engine and have their burners annularly mounted at one end of the chamber. The hot combustion gases then pass through a low-pressure turbine 25 before passing through a heat recovery steam generator. In order to generate electricity, the compressor 21 and turbines 23,25 drive a generator 26 via a shaft 30.
The burner of the high-pressure combustor is typically a pre-mix burner 20 as shown in FIG. 2. It typically includes a conical swirl shaped body in the form of a double cone 11 which is concentric with the burner axis, wherein the region between the double cone 11 body and burner axis defines a swirl space 17. A central fuel lance 12 lies within the burner axis extending into the swirl space 17. In a first stage 18, pre-mix fuel is injected radially into the swirl space 17 through injection holes in the fuel lance 12 while in a second stage 14, pre-mix fuel is injected through injection holes located in the double cone 11 section of the burner into an air stream conducted within the double cone 11.
In order to facilitate the inspection of a gas turbine engine, each of the combustors is separable along a split plane, forming a split line. The need to break the combustors at the split line means that the split line cannot be totally sealed, resulting in gas leakage of cooling gas from the plenum surrounding the combustor. This leakage results in localized cooling that extends to adjacent burners, resulting in greater formation of CO in these burners than in other burners. When the cooling gas is air, the localized cooling is coupled with increased oxygen concentration, exacerbating the problem.
CO is a restricted gas for emission purposes and so there is a desire to reduce its production. While operating parameters, such as combustor inlet and flame temperature, impact the formation of CO, due to the overriding need to drive engine throughput and efficiency it is undesirable to use these parameters as CO emission control parameters. There is therefore a need for an alternative.